Text processing is an old problem in computer science, most programming tutorial do some form of text manipulation – if only to send back an “Hello World” back to the user. This does not mean that it is an easy problem, and there are many misconception about text and its representation. Here is a list – it is certainly not exhaustive and probably not original, just stuff I noticed (and that I need to get off my chest).

English text can be transcribed using American Standard Code for Information Interchange (ASCII).

ASCII has no representation for the thorn letter (Þ) the long S (ſ), and the tréma (like in naïve).

Modern english text can be transcribed using ASCII.

Typographic quotes “ and ” cannot be represented in ASCII.

C programs can handle ASCII text with no problem.

An ASCII text can contain the NULL (0x00) character in its midst. Many C programs cannot handle this.

Languages like C require ASCII

For a long time, the C language offered trigram facilities to encode characters that were not available in EBCDIC.

ASCII Does not encode code-points above 127

ANSI escape sequences allow for encoding C1 control characters, which are in the 128 – 159 range.

ASCII data represent text

The bell character does not represent textual information, but sound (or blinking the screen).

Interpreting ASCII data is easy.

ASCII data specifies many obscure control characters in the 0 – 31 range that nobody knows how to interpret.

Interpreting common ASCII data is defined

The width of white space represented by a tab character (7) is not defined. The way carriage return (13) and line-feed (10) are used in text-files is still a matter of debate.

An ASCII text is valid UTF-8

ASCII data is always structurally valid UTF-8, it is not guaranteed to be interchange valid UTF-8, as many ASCII control characters like bell (0x07) are not defined in Unicode.

A Unicode file that only contains ASCII letters, encoded in UTF-8, is a valid ASCII file.

Unicode allows for an optional starting Byte Order Marker (BOM) character at the start of the stream. Encode in UTF-8 this will be the sequence EFBBBF which is not valid ASCII. This causes some issues when moving data from system that recognise the BOM (Windows) to systems that do not (Unix).

ISO-Latin-1 can represent Western European text

The french o-e ligature character (œ) cannot be represented in ISO-latin-1 (ISO 8859-1), nor can the german Capital sharp s (ẞ).

Windows uses ISO-Latin-1 encoding

Windows often uses the Windows-1252 encoding, which is a variation of ISO-Latin-1 which ads more western latin characters in the range occupied by the C1 control characters in ISO-Latin-1.

ISO Latin-1 can be mapped directly into Unicode

This is true for real ISO Latin-1, but given the confusion between ISO Latin-1 and Windows-1252, HTML5 now recommends to interpret ISO Latin-1 as Windows-1252, so the code-points in the C1 range need to be re-mapped.

Text can be represented in one way in Unicode

Accented character like ‘é’ typically have two representations, one where the accent is composed with the letter, here with code point U+00E9, or decomposed into a letter and an accent, here the sequence U+0065 – U+0301.

Character decomposition is latin script problem

Korean Hangul also has decomposed forms

English text can be represented in one way in Unicode

Unicode has ligature characters for the sequences ff, fi, fl, ffi, ffl. These are output by tools like LaTex, even for english.

Each ASCII character displayed with a mono-space font has the same width

The tab character can have the width of multiple characters.

Ignoring control and combining characters, Unicode characters have the same width with a mono-space font

Asian character (often named full-width) are typically wider (typically 50%) that latin letters, even in mono-space fonts. See for instance:
Cat
日本語

Full-width characters only represent Asian characters

There is a full-width version of the ASCII range U+0021 to U+0073 starting from U+FF01.

Non control characters are either half-width or full-width

The character ﷽ (Arabic Ligature Bismillah ar-Rahman ar-Raheem) (U+FDFD) is very wide.

A terminal can display Unicode text

Terminals are typically character oriented, so even if it is set up in UTF-8 mode, all fonts are installed, scripts which rely on ligatures (like Arabic), will not display properly.

UTF-8 can be safely manipulated at the byte level

Any substring operation that cuts a multi-byte character will yield invalid UTF-8.

Language like Java have Unicode support

Java and Javascript were designed around the now abandoned UCS-2 encoding, which assumes that all Unicode characters will fit on 16 bits, i.e. the U+0000 – U+FFFF range. They don’t. Java and JavaScript now uses the UTF-16 encoding, which means their character type might represent a fraction of a character (surrogate pairs).

Splitting at code-point boundary is safe

In general, cutting between a combining character and the character it combines with (say an accent), will not yield the expected result. In UTF-16, you might cut surrogate pairs. Certain ranges, like the Emoji flag range are only defined for character pairs.

A Unicode Code-point can only represented in one way in UTF-8

In theory this is true, but some systems wrongly convert UTF-16 into UTF-8 by encoding surrogate characters directly, instead of decoding them and encoding them as UTF-8. Some UTF-8 decoders accept this mistake. They might also decode UTF-8 encoded Windows-1252 code-points into their respective characters.

Unicode does not handle formatting

Even if you ignore ANSI escape sequences (which allow underlining and bolding of text), variation selectors (which select the colour / black and white version of Emojis), there are ranges that duplicate ASCII with various style attributes (italic, bold). I wrote a markless, a tool that allows to render Markdown data using just Unicode tricks.

There is a standard to encode Unicode characters in ASCII

There are many, HTML has three (named entities, hex-encoded, decimal encoded), The C programming language has one (based on UTF-8), C99 has another. URL and CSS use different schemes, and so do e-mails.

ASCII and its variants can be mapped into Unicode

Some obscure variants like PETSCII contain graphical characters that are not mapped (yet).

Unicode data is meant to be represented in black and white

Emoji characters are typically in colour.

Unicode data is meant to be represented visually

The range U+2800 to U+28FF contains braille patterns.